According to Morten: Get latest version of oplot, from ftp://kaktus.kemi.aau.dk/pub/o/oplot. The background colour is specified with a switch -c, and r,g,b values, for example:
oplot -fc"0.7 0.56 1.0" < o_008.plt > o_008.ps
The r,g,b value must be enclosed in double quotes, and separated by spaces or commas. No compiled files can be found yet, you have to compile yourself. I have tested the new option on HP, SGI and ESV.